1. Field of the Invention
The present invention relates to a method of and an apparatus for continuously irradiating a running metal strip with an electron beam.
2. Description of the Related Art
Electron beam radiation has been used to improve characteristics of a metal strip, particularly to reduce iron loss in electromagnetic sheet sheet. The surface of a metal strip such as an electromagnetic steel strip, whether coated with ceramic or not, has been irradiated. This technique is disclosed, for example, in Japanese Patent Laid-Open Nos. 63-96218 and 63-186826.
A strongly condensed electron beam is applied to form linear traces which extend across the strip and which are arranged at a predetermined pitch in the longitudinal direction of the strip. The period of time of beam application for forming a single linear trace, and its pitch, are determined on the basis of the amount of input energy which provides maximum improvement in the characteristics of the strip.
Usually an electron beam gun is used as the source of the electron beam. It can perform a very quick scanning deflection of the beam by virtue of a deflection coil which also is known as a "deflection lens" or "electronic lens" but the beam gun cannot repeatedly turn the beam on and off at high frequency. Consequently, it has been necessary to apply the irradiation in a continuous mode.
In applying the electron beam technique to a practical production line, in ordinary cases, the strip velocity is determined on the basis of the production rate. When irradiation with an electron beam is applied, however, the velocity of the strip is controlled and limited on the basis of product quality, since the irradiation time for forming one linear trace and the pitch of the irradiation ar important in controlling the quality of the product.
Turning to FIG. 3 of the drawings, the optimum beam scanning speed is expressed as S and the scanning pitch is expressed as P. The irradiation width is expressed as W. The time Ts required for forming one linear trace is: EQU Ts=W/S (1)
If the strip is conveyed through a distance P, the strip can be steadily or continuously irradiated. In such a case, the line velocity v is determined as: EQU v=P/Ts=(P/W).multidot.S (2)
Thus, for continuous irradiation, the strip should be conveyed at the line speed "v," which limits the production rate. Consequently, the production rate must be controlled by the total working hours of the production line, i.e., on the basis of the number of days on which the operation is suspended. This undesirably involves wasteful use of the line driving system and requires complicated adjustment of schedules of the line operators.